Biomolecules--that is, molecules of compounds which engage in a biological activity or are effective in modulating a biological activity--commonly are used in solution or are adsorbed or otherwise attached to solid supporting surfaces such as glass beads. U.S. Pat. No. 3,959,078 refers to the attachment of enzymes to solid surfaces.
Biomolecules can alter the solid or semi-solid surfaces to which they are attached. Heparin, for example, can be attached to polyethylene surfaces of a blood bag to provide the surface with anticoagulant properties. Ebert et al.; The Anticoagulant Activity of Derivatized and Immobilized Heparins, in Biomaterials: Interfacial Phenomena and applications, Cooper et al., eds., Am. Chem. Soc. 1982, pp. 161-176.
Several procedures currently exist for immobilizing synthetic and naturally produced molecules onto solid or semi-solid substrates. The chemistries typically utilized are either highly substrate dependent or result in a significantly reduced activity of the immobilized species. An example of such chemistries include copolymer grafting procedures Larsson, P. H. Johansson, S. G.; Hult, A.; and Gothe, S.: Covalent Binding of Proteins to Grafted Plastic Surfaces Suitable for Immunoassays; J. Immuno. Methods, 98, 1987, p-129-135.
The tertiary and quaternary structures of such biomolecules as proteins and polysaccharides have historically been viewed as being "static" in nature. This static view of biomolecular function recently has given way to understanding the dynamic motions of intramolecular structures as a basis for function. Jarplus, M.; McCammon, J. A.: The Dynamics of Proteins, Scientific American, April, 1986, p. 42-51. An implication of this understanding is that for optimal activity, proteins and other biomolecules should be immobilized by methods that distort neither the conformation nor the molecular motions of the biomolecules.
In addition to the loss of activity, the conformational distortion that may occur upon immobilization of a biomolecule can give rise to undesired biological responses especially on implant and medical device surfaces. For instance, increased thrombogenicity or induction of foreign body reactions and rejection following implantation have been reported. When certain macromolecules (e.g. proteins and polysaccharides) encounter previously untreated polymers or other medical device materials, they may adsorb onto these surfaces and undergo alterations in both conformation and activity. Foreign body reactions to soft tissue implants and the thrombogenicity of most polymers involve a cellular level response in the host after a layer of protein has adsorbed onto the device surface. The so called adverse host response may be attributed to the altered macromolecule structure producing an abnormal function when immobilized on the device's surface.
Various plasma proteins undergo slow conformational changes resulting in the loss of secondary structures, such as altered helix or beta sheet. The denaturation of surface immobilized proteins through these type of changes may render them antigenic. Peters, J. H. and Goetzl, E.: Recovery of Greater Antigenic Reactivity in Conformationally Altered Albumin, J. Biological Chem., 224, 1969, p. 2068; Stern, I. J., et al.: Immunogenic Effects of Materials on Plasma Proteins, Conf. Proc. Artificial Heart Program, National Heart Institute, 1969, p. 259. The immobilization of some plasma proteins which may produce an altered thrombogenic response. Brash, J. L.; Protein Interactions with Artificial Surfaces, Interaction of the Blood with Natural and Artificial Surfaces, ed. Salzman, E. W., 1981 by Marcel Dekker, Inc., p.39-44.
By spacing biomolecules away from a support, it would be expected that somewhat improved biomolecule activity should be observed. Initial studies indicating the utility of spacer arms have been conducted with heparin. Ebert et al., supra, reported that the bioactivity of heparin could be correlated, to a limited degree, with the length of a spacer holding heparin molecules apart from a supporting surface. Activated partial thromboplastin time was assayed with bovine plasma as an indicator of heparin activity. Heparin was immobilized with hydrophobic aliphatic spacers of varying lengths and produced heparin activity that increased with spacer length. However, because of the hydrophobic nature of alkane spacers, the use of longer chains under aqueous physiological conditions would be expected to result in a coiling or doubling back of the spacer molecules, the spacers thus losing their ability to space biomolecules from a solid or semi solid surface.